Precipitators have long been used to remove dust particles from combustion gases and air. A particularly important application for precipitators is found in electric power generating plants. Electric power generating plants burn coal to create steam to power generators. The combustion gases from the coal are laden with fly ash. The fly ash must be removed from the gases before the gases are released to the atmosphere through the power plant chimney. Precipitators are the devices used to remove this fly ash.
Electrostatic precipitators produce an electric charge on the particles to be collected and then propel the charged particles by electrostatic forces to collecting plates. A discharge electrode or cathode is set up between collecting plates or the anode. An intense, high voltage electrical field is maintained between the discharged electrode and the collecting plates. The flue gas carrying the fly ash is ionized by the intense, electric field. The gas ions charge the entrained particles. The negatively charged particles still in the presence of the electrostatic field are attracted to the positively charged collecting plates. Periodically rappers come down and bang the collecting plates to loosen and drop the collected particles down into storage hoppers. The collection efficiency of the electrostatic precipitator is related to the time the particles are exposed to the electrostatic field, the strength of the field, and the resistivity of the particles. Therefore it is desirable to keep the strength of the electrostatic field as high as possible or maintain the maximum differential between the cathode and the collection plates.
When a certain differential is attained sparking will begin to occur. Sparking is a luminous discharge which occurs as a result of the omission of electrons at high potentials. The computer center controlling the voltage in the cathode wires uses TR sensors to count the number of sparks per minute in a field. For example they may be set for a count of 20 to 40 sparks per minute. If there are less than 20 sparks per minute then the precipitator is not running at maximum efficiency (typically 99.7% efficiency is the goal in a precipitator). At a certain voltage a corona will appear. A corona is a luminous discharge in the space surrounding a high voltage conductor caused by ionization of the gas medium. The discharge constitutes a loss of energy. The corona being caused by the emission of electrons is dependant upon the curvature of the conductor surface, with most emission occurring from sharp points and the least from surfaces with a large radius of curvature. The corona is usually accompanied by a blue glow and a crackling or hissing sound.
Once voltage begins to exceed preferred levels a dangerous potential is being reached which can lead to arcing. An arc is a sustained luminous discharge between the cathode and the collection plates. Because it is sustained rather than intermittent, an arc is distinguished from a spark discharge (the later being a series of discharges or sparks even when it appears continuous). An arc can actually puncture through a material such as the collection plates. So once again to prevent arcing the control unit is trying to minimize sparking and still maintain efficiency.
Now keeping in mind that corona and arcing are dependent upon the radius of curvature of the anode and cathode and upon the potential between the cathode and the collection plates some of the problems encountered in maintaining a precipitator at maximum efficiency will be appreciated. For instance, the fly ash building up on the collection plate may not build up uniformly or some of the fly ash accumulates on the plates irregardless of rapping. This build-up and accumulation will create points which increase the propensity for arcing. Depending upon the type of coal being burned the ash build up will effect the conductivity or resistivity of the medium between the cathode collection plates.
The accumulated ash essentially creates two voltage drops, one between the cathode and the surface of the build-up and the other between the surface of the build-up and the collection plate. When the later potential reaches a critical value a back corona or a positive corona off of the collection plate can be created. Positive coronas are very unstable and result in poor efficiency of the precipitator. Any edge created on the collection plate will significantly increase the propensity for arcing between the wires and the collection plate and the voltage of the entire field must be limited to prevent the arcing. If you have a point or edge no matter how minute it's going to increase the propensity for arcing.
Another factor which needs to be considered is that different coals burned in the power plants have different characteristics. For instance some coals have more sulphur than others. When this coal burns the sulphur gets on the fly ash and acts as a conductor. Coals also include sodium which also may act as a conductor. Soda ash can be injected into the flue gas to increase the amount of sodium. Sulphur can be injected as a technique to control resistivity. So one can see that the composition of the coal and therefore the composition of the fly ash affects the resistivity of the electrostatic field.
The larger the resistivity, the greater the propensity to get a back corona. This is because the accumulated ash will create a larger voltage drop due to a high resistivity layer. On the other hand one does not want the resistivity to be too low because the ash build-up should be somewhat sturdy so that when the rapper strikes the collection plate the packed fly ash will drop as a sheet into the hopper rather than vortexing back into the gases passing through the precipitator and out the chimney.
As an example plates may be 30 feet high and 10 feet wide. The ash building up on the plate may build up excessively from, for example, a half to one inch thick. If the ash is correctly packed and rapped the sheet of ash will fall down into the hopper. A little bit of the ash will move back into the flue gas and move downstream to the next precipitator in series. However the rapping must be controlled since it can damage the plate or the wires or if the rapping occurs too soon the accumulated ash may not properly pack. Sulphur dioxide contained in the ash combined with moisture causes corrosion which the rappers won't take off. Over time the ash accumulates on the surface of the plates.
The collection plates are electrically connected in fields with typically three to five fields in series. If the first field captures 70% of the fly ash then the second field theoretically would capture 70% of the remaining 30% which is 21%. Now with the remaining 9%, 70% of it is captured by the third field and the remaining 2 to 3% is reduced by 70% in the last precipitator before the gasses pass out of the chimney. So the first set of plates will be collecting a much higher volume than the outlet plates and therefore will be the dirtiest.
It must also be considered that resistivity is a function of temperature and since a certain amount of resistivity is desired, depending upon the type of coal or coal chemistry, the flue gas temperatures flowing through the precipitators may be controlled so that a desired resistivity is obtained.
Because of ash accumulations in the precipitator and the resulting effect upon the efficiency of the precipitator, precipitators should periodically be cleaned. Going back about eight years or so most of the precipitators were water washed meaning they would either be cleaned by hydroblasting or with a high pressure fire type hose. One of the problems with water washing is that all of the water ends up dropping into the hoppers. In most cases hoppers use a dry pneumatic system to move out the ash. Wet fly ash becomes hard like cement which can damage this dry pneumatic system. Procedures required to prevent water from entering the system are very time consuming, very labor intensive and expensive. In most cases the water washing of a precipitator including dry out time could not be completed in a weekend. Water washing requires much more time than blasting. Another problem was the water combining with the sulphur and the ash to create sulfuric acid. This lead to corrosion and rust problems on the plates. Water washing is still used in some plants.
Because of the long periods of down time for cleaning the precipitators, the business was previously seasonal. Extreme temperature variations of the summer and winter create peak demands preventing shutdown for extended periods. So there were two time periods per year that the majority of the cleaning work would be done.
Within the last few years Applicant utilized sand blasting for the cleaning of precipitators. Sandblasting eliminated the above-discussed problems that are encountered with water washing. However sand blasting has other problems. Sand contains a large amount of free silica and therefore when the sandblasting was carried out large amounts of free silica dust was generated. Free silica is a hazardous substance which is known to cause silicosis and which is strictly regulated by governmental agencies. Sand is also very abrasive. The blasting velocities needed to clean the precipitators combined with the abrasivity of sand scores the plates and may blow holes in the plates. The abrasion of the plate creates tiny peaks which the ash builds upon to increase the propensity of sparking. One small hole in one collection plate will affect an entire electrostatic field because it will be the point which begins to arc first. The collection plates are very thin, anywhere from 16 to 22 gauge. It's dark and dusty when a worker is actually in the precipitator cleaning the collection plates so it's very difficult to see what is being blasted while you're trying to get the entire plate clean. It is much easier to destroy to wear the proximal end of the plate than to destroy or wear the distal end of the plate although both must be equally clean. You also get a build-up on the wire, which may for example be one eighth inch wire, which is called "donuts." The build-up and blasting of the wires can lead to breaking of the wires which also limits or kills the entire electrostatic field. Lastly, the sandblasting technique leaves a sandy film or residue on the wires or collection plates. Since sand is relatively resistive this resistive residue layer enhanced arcing and back corona related problems.
The use of grains and walnut shells in the blasting of precipitators has also been used within the last few years. The use of this technique is considered to be very dangerous because of grain dust build-up which upon ignition could create an explosion.
There has also been prior art attempts to use carbon dioxide, dry ice in the frozen state. This simply did not work because the dry ice would evaporate before it could clean the collecting plates.
The need therefore existed for a method of cleaning precipitators which could be carried out quickly, thoroughly, without releasing large amounts of free silica, without damaging (abrasion or corrosion) the wires and collection plates, without the release of combustible dust, and which relatively speaking would leave a resistivity less than that of sandblasting.